In recent years, thermal power equipment and boiler equipment have used large amounts of coal, heavy oil or superheavy oil as fuels. From the points of view of air pollution control and Earth environment purification, it has become problems how to decrease the quantities and concentrations of emissions of sulfur oxides (mainly sulfur dioxide), nitrogen oxides, and carbon dioxide. Suppression of carbon dioxide emission, in particular, has recently been investigated, together with emission control of flon gas and methane gas, from the viewpoint of global warming. For this purpose, methods for removing carbon dioxide, such as PSA (pressure swing) method, membrane separation, and absorption by reaction with basic compounds, are under study.
As an example of a method for removing carbon dioxide with the use of basic compounds, Japanese Unexamined Patent Publication No. 1993-184866 (related U.S. Pat. No. 5,318,758) proposes a method which performs decarbonation by using an aqueous solution of an amine compound (hereinafter referred to simply as an amine) as a solution for absorbing carbon dioxide. In this method, the reaction between carbon dioxide and the amine compound is an exothermic reaction. Thus, the temperature of the absorbing solution in a carbon dioxide absorption section rises to raise the vapor pressure of the amine. That is, the amine-containing absorbing solution evaporates owing to the temperature increase. As a result, the amount of the amine compound accompanying a decarbonated gas increases. Thus, a water washing section is provided in an absorption tower, and the decarbonated gas and washing water are subjected to vapor-liquid contact in the water washing section, whereby the amine compound accompanying the decarbonated gas is recovered into a liquid phase.
Concretely, the above-mentioned Japanese Unexamined Patent Publication No. 1993-184866 discloses a decarbonation apparatus as shown in FIGS. 2 and 3.
In FIG. 2, the reference numeral 1 denotes an absorption tower, 2 a carbon dioxide absorption section, 3 a water washing section, 4 an exhaust gas supply section, 6 is an absorbing solution supply port, 7 a nozzle, 8 a liquid reservoir in the water washing section, 9 a circulating pump, 10 a cooler, 11 a nozzle, 12 an absorbing solution discharge port, 13 a blower, 14 an exhaust gas supply port, 15 an exhaust gas cooler, 16 a circulating pump, 17 a cooler, 18 a nozzle, and 19 a drainage line.
Although a detailed explanation is omitted, a combustion exhaust gas supplied through the exhaust gas supply port 14 is cooled by the cooling tower 15, and then fed to the absorption tower 1. In the carbon dioxide absorption section 2 of the absorption tower 1, the fed combustion exhaust gas is brought into countercurrent contact with an absorbing solution supplied through the absorbing solution supply port via the nozzle 7. As a result, carbon dioxide in the combustion exhaust gas is absorbed and removed by the absorbing solution. The loaded absorbing solution, which has absorbed carbon dioxide, is sent to a regeneration tower (not shown) through the absorbing solution discharge port 12. In the regeneration tower, the loaded absorbing solution is regenerated, and fed again from the absorbing solution supply port 16 to the absorption tower 1.
On the other hand, the combustion exhaust gas decarbonated in the carbon dioxide absorption section (i.e., decarbonated exhaust gas) ascends, accompanied by a large amount of an amine vapor, due to a temperature rise ascribed to an exothermic reaction between carbon dioxide and an amine compound in the carbon dioxide absorption section 2. The ascending decarbonated exhaust gas passes through the liquid reservoir 8, and heads toward the water washing section 3. In the water washing section 3, reserved water in the liquid reservoir 8 is transported by the circulating pump 9, cooled by the cooler 10, and then supplied to the water washing section 3 as washing water through the nozzle 11. As a result, this washing water and the decarbonated exhaust gas make countercurrent contact in the water washing section 3, whereby the amine compound in the decarbonated exhaust gas is recovered into the liquid phase.
FIG. 3 is characterized by improving the amine recovering ability by utilization of regeneration tower refluxed water. In FIG. 3, the reference numeral 21 denotes an absorption tower, 22 a carbon dioxide absorption section, 23 a water washing section, 24 an exhaust gas supply port, 25 an exhaust gas discharge port, 26 an absorbing solution supply port, 27 a nozzle, 28 a regeneration tower refluxed withdrawn water supply port, 29 a nozzle, 30 a cooler, 31 a nozzle, 32 a charging section, 33 a circulating pump, 34 a make-up water supply line, 35 an absorbing solution discharge pump, 36 a heat exchanger, 37 a cooler, 38 a regeneration tower, 39 a nozzle, 40 a lower charging section, 41 a reboiler, 42 an upper charging section, 43 a refluxed water pump, 44 a carbon dioxide separator, 45 a carbon dioxide discharge line, 46 a cooler, 47 a nozzle, 48 a refluxed water supply line, and 49 a combustion gas supply blower.
Although a detailed explanation is omitted, a combustion exhaust gas supplied by the combustion gas supply blower 49 is cooled by the cooling tower 30, and then fed to the absorption tower 21. In the carbon dioxide absorption section 22 of the absorption tower 21, the fed combustion exhaust gas is brought into countercurrent contact with an absorbing solution supplied through the absorbing solution supply port 26 via the nozzle 27. As a result, carbon dioxide in the combustion exhaust gas is absorbed and removed by the absorbing solution. The loaded absorbing solution, which has absorbed carbon dioxide, is sent to the regeneration tower 38 by the absorbing solution discharge pump 35 through the absorbing solution discharge port 12. In the regeneration tower 38, the loaded absorbing solution is regenerated, and fed again to the absorption tower 21 through the absorbing solution supply port 26.
On the other hand, the combustion exhaust gas decarbonated in the carbon dioxide absorption section 22 (i.e., decarbonated exhaust gas) ascends, accompanied by a large amount of an amine vapor, owing to a temperature rise ascribed to an exothermic reaction between carbon dioxide and an amine compound in the carbon dioxide absorption section 22. The ascending decarbonated exhaust gas heads toward the water washing section 23. In the water washing section 23, part of regeneration tower refluxed water withdrawn as washing water is supplied to the water washing section 23 through the regeneration tower refluxed withdrawn water supply port 28 via the nozzle 29. As a result, this washing water and the decarbonated exhaust gas make countercurrent contact in the water washing section 23, whereby the amine compound in the decarbonated exhaust gas is recovered into the liquid phase.
However, according to the above-described conventional decarbonation apparatus shown in FIG. 2, in particular, the water washing section is provided as one stage. Thus, the concentration of amine recovered by the washing water is so high that the recovery of amine is insufficient. As a result, amine accompanies the decarbonated exhaust gas, and is released to the outside of the decarbonation process system. Consequently, amine is wasted, causing a concern about an increase in the operating cost, etc.
The present invention has been accomplished in the light of the foregoing problems. Its object is to provide an amine recovery method and apparatus, and a decarbonation apparatus equipped with the amine recovery apparatus, the amine recovery method and apparatus being capable of efficiently recovering an amine compound accompanying a decarbonated exhaust gas in a decarbonation process in which carbon dioxide is removed from a gas containing carbon dioxide with the use of an amine compound-containing absorbing solution.